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A converging beam of light forms a sharp...

A converging beam of light forms a sharp image on a screen. A lens is placed in the path of the beam at `10 cm` from the screen. It is found that the screen has to be moved `8 cm` further away from the lens to obtain a sharp image. Find the focal length and nature of the lens.

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The correct Answer is:
(a) f=22.5cm ; (b) concave lens
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